A conventional mechanical crash detection sensor, sometimes called a safing sensor, typically consists of an acceleration sensing device that biases closed a set of normally open mechanical switch contacts at a predetermined rate of deceleration, thereby completing an electrical circuit. Alternatively, the sensor may switch on a binary output using a conventional electronic switching circuit. Examples of such sensor arrangements may be found in U.S. Pat. No. 4,995,639 to Breed.
A safing sensor is employed in an airbag inflation circuit in an analogous fashion to the safety mechanism of a firearm. The safing sensor, upon detecting a predetermined threshold level of deceleration just below that of an automobile crash event, switches on a binary output or closes a set of normally open switch contacts. The output or switch contacts are interposed in series with an airbag inflator circuit such that the inflator circuit is armed (ready to fire) when the output is on or the switch contacts closed. The inflator circuit typically one or more mechanical or electronic crash discrimination sensors in conjunction with at least one safing sensor to determine the precise moment to initiate airbag inflation.
Therefore, a mechanical safing sensor will switch on many times during its normal service life under conditions that are normally insufficient to fire a crash discrimination sensor. Due to the criticality of airbag deployment in the event of a crash, it is generally desirable to be able to diagnose mechanical sensors for non-functional conditions. These conditions may include mechanical switch contacts being "stuck open", or "stuck closed", corroded contacts which may cause high resistance in an airbag inflation circuit, or an inoperative binary output.
Previously, only specially designed mechanical sensors have provided the diagnostic capability to detect "stuck open" or "stuck closed" contacts. More specifically, diagnostic capabilities have previously been achieved for "stuck open" contact situations by using an electromagnetic coil, a magnet, and several electronic components to supply current to the coil. When electrical current is applied to the coil, the electromagnet biases the mechanical contacts closed thereby causing a change in resistance as measured across the contacts. By monitoring the operation of the contacts during the operation of the testing circuit, proper operation of the sensor may be detected.
However, while such a sensor testing arrangement provides diagnostic capability, it is a very costly method of gathering information about the operational status of a mechanical sensor, both in the requisite number of components and the relative complexity of assembly. Furthermore, these known diagnostic mechanical sensors also suffer from the disadvantage that the testing circuit may perform a self-test at an inopportune time and thereby initiate inadvertent inflation of the airbag.